Gnosis: The First Instrument to Use Fiber Bragg Gratings for Oh Suppression

Abstract

The near-infrared is an important part of the spectrum in astronomy, especially in cosmology because the light from objects in the early universe is redshifted to these wavelengths. However, deep near-infrared observations are extremely difficult to make from ground-based telescopes due to the bright background from the atmosphere. Nearly all of this background comes from the bright and narrow emission lines of atmospheric hydroxyl (OH) molecules. The atmospheric background cannot be easily removed from data because the brightness fluctuates unpredictably on short timescales. The sensitivity of ground-based optical astronomy far exceeds that of near-infrared astronomy because of this long-standing problem. GNOSIS is a prototype astrophotonic instrument that utilizes "OH suppression fibers" consisting of fiber Bragg gratings and photonic lanterns to suppress the 103 brightest atmospheric emission doublets between 1.47 and 1.7 µm. GNOSIS was commissioned at the 3.9 m Anglo-Australian Telescope with the IRIS2 spectrograph to demonstrate the potential of OH suppression fibers, but may be potentially used with any telescope and spectrograph combination. Unlike previous atmospheric suppression techniques GNOSIS suppresses the lines before dispersion and in a manner that depends purely on wavelength. We present the instrument design and report the results of laboratory and on-sky tests from commissioning. While these tests demonstrated high throughput (≈ 60%) and excellent suppression of the skylines by the OH suppression fibers, surprisingly GNOSIS produced no significant reduction in the interline background and the sensitivity of GNOSIS+IRIS2 is about the same as IRIS2. It is unclear whether the lack of reduction in the interline background is due to physical sources or systematic errors as the observations are detector noise dominated. OH suppression fibers could potentially impact ground-based astronomy at the level of adaptive optics or greater. However, until a clear reduction in the interline background and the corresponding increasing in sensitivity is demonstrated optimized OH suppression fibers paired with a fiber-fed spectrograph will at least provide a real benefit at low resolving powers.

Ultrafast-laser-inscribed 3D integrated photonics: challenges and emerging applications

“…Furthermore, because the star light is carried in single-mode optical fibres, other photonic technologies can be applied, such as fibre Bragg gratings for narrow-band spectral filtering of detrimental background light. This has been successfully applied to suppress 148 bright emission lines arising from hydroxyl groups in our Earth's atmosphere [118]. However, fibrebased photonic lanterns are difficult to scale to higher number of single-mode fibres and are labour intensive to fabricate.…”

PRAXIS: low thermal emission high efficiency OH suppressed fibre spectrograph

“…Observations with GNOSIS 6,8 showed that although strong OH suppression was achieved by the FBGs, the overall sensitivity of IRIS2 was not improved. This was due to two reasons: the low overall throughput of GNOSIS + IRIS2 meant that below 1.6 µm the background was dominated by the detector dark current, and above this the background was dominated by thermal emission from the warm relay optics.…”

Photonic lanterns

“…Thus, a perfect lantern device has an intrinsically limited bandwidth, since the number of modes could only be perfectly matched over a restricted wavelength ranges determined by the modal cut-off of the multimode waveguide at those wavelengths and the number of single-mode waveguides. Devices with a 300 nm bandwidth have been already demonstrated with a minimum 85% transmission at the edges of the usable wavelength span [13,22].…”

“…An OH-sky suppression instrument called GNOSIS [22], installed at the 3.9 m Anglo Australian Telescope (AAT) in Australia ( Figure 5A), was made possible by photonic lantern technology. This instrument efficiently transfers the light captured and coupled into multimode fibers by the telescope into single-mode fibers through a photonic lantern.…”